1. Field of the Invention
The invention relates to aircraft flight path control apparatus particularly with respect to flight path control in the transition from a turn maneuver to heading/track hold.
2. Description of the Prior Art
Selection of heading or track modes for an aircraft is performed by the flight control panel. The panel is configured with various selector knobs and digital display windows for use by the pilot in manually selecting desired flight paths for autopilot or flight director control. Two of these flight paths are known as heading and track. When flying heading, the aircraft is commanded to point headin at a specific angle with respect to magnetic north. When controlled to track, the aircraft is commanded to fly in a specific terrestrial direction, regardless of crosswinds. In the absence of a crosswind, heading and track will follow an identical course under normal conditions.
The autopilot includes provision for a heading or track hold mode in which the desired direction will be maintained after execution of a turn maneuver. When the aircraft operator decides to terminate a turn maneuver and hold heading or track, it is desirable to display the new heading or track in the window of the control panel at the moment the heading or track hold mode is selected. This allows the operator to immediately visualize where the aircraft will be located before waiting for a roll-to-wings level maneuver to be completed. The prior art held the aircraft to the existing heading at the time the hold mode was selected. This resulted in an undesirable overshoot in heading and roll. The present invention provides a heading or track correction, based on the predicted path of the aircraft, with known inputs from the onboard navigation systems. The heading/track hold predict algorithm uses roll angle and roll attitude rate from an inertial reference system and estimated true airspeed from the flight control computer to predict the new heading or track which the aircraft will experience. The model used to develop the algorithm assumes the roll rate of the aircraft approaches a maximum roll rate exponentially with a predetermined time constant and then at some point in time approaches zero degrees of roll angle exponentially with the same predetermined time constant. The change in heading is based upon this assumed exponential path to wings level during a coordinated turn.